To reach their findings, the researchers looked at nearly 2,000 individuals between the ages of 20 and 49.

The nationwide study, which was conducted between 2001 and 2004, compared vitamin D levels in each participant’s blood with their cardiorespiratory fitness, which was measured by a treadmill test.

Participants were then divided into quartiles, or four equal groups, based on their vitamin D levels.

The cardiorespiratory fitness for individuals in the top quarter of participants was 4.3 times higher than people in the bottom quartile.

Even after adjusting for factors that can influence this relationship — age, sex, race, body mass index, hypertension, diabetes, and smoking history, for example — the link remained significant.

The top quarter of participants still had a cardiorespiratory fitness 2.9 times greater than people in the bottom section.

“The relationship between higher vitamin D levels and better exercise capacity holds in men and women, across the young and middle age groups, across ethnicities, regardless of body mass index or smoking status, and whether or not participants have hypertension or diabetes,” Dr. Amr Marawan, assistant professor of internal medicine at Virginia Commonwealth University and a co-author of the study, said in a statement from the European Society of Cardiology (ESC).

Each 10-point increase in vitamin D levels in the blood was associated with a 0.78-point increase in V02 max.

This suggests that incremental increases in vitamin D equal an equivalent improvement in your exercise capacity.

Marawan is quick to point out that their research is only an observational study. It can’t show a cause-and-effect relationship between vitamin D and exercise capabilities.

However, he added in the statement to ESC that the association between the two factors is “strong, incremental, and consistent across groups.”

Vitamin D, also known as the sunshine vitamin, develops naturally in your body in response to sun exposure.

It’s also found naturally in some foods, such as fish and egg yolks.

While your body can make vitamin D naturally, it can only do so during adequate sun exposure. That amount of sunlight is difficult to find in winter months and in latitudes above the 37th parallel north (a line that runs roughly through the United States from San Francisco to New York).

In these months, it’s better to look for the vitamin in fatty fish, including tuna, mackerel, and salmon, as well as egg yolks, fortified cereal and milk, and cheese. Some leafy greens, such as spinach, kale, and collard greens, are also good sources of vitamin D.

Supplements can be used to boost your levels of this important vitamin, too.

“The daily recommended dose is 400 to 800 international units (IU),” Marawan told Healthline. “There are some studies supporting higher doses up to 4,000 IU for better cardiovascular health, but more studies didn’t support this. There is no agreement that higher vitamin D supplements will have a better outcome.”

But you have to use caution with supplements.

Too much vitamin D can lead to toxicity. Symptoms of vitamin D toxicity include vomiting, nausea, and weakness.

The study from Marawan and colleagues isn’t the first research to connect vitamin D levels to heart health and exercise capacity.

In 2011, Harvard researchers released a study that found that people who exercise vigorously and regularly have higher levels of vitamin D.

A 2017 study from Johns Hopkins University in Maryland found a “synergistic” link between exercise and good vitamin D levels, too. The researchers determined that people who exercise more have higher levels of vitamin D, and the most active study participants with the highest levels of vitamin D also had the lowest risk of cardiovascular disease.

From all these studies, however, it’s still unclear whether vitamin D is driving fitness, or if increased fitness helps the body boost vitamin D stores.

It appears instead that the two work together to equally enhance one another.

That means that taking more vitamin D alone isn’t going to improve your VO2 max or fitness capabilities. You have to exercise to reap the benefits.

“Physical activity and fitness are correlated, so it is not surprising to me that vitamin D would be associated with fitness,” Dr. Erin D. Michos, MS, associate professor of medicine and epidemiology and associate director of preventive cardiology at Johns Hopkins University School of Medicine, told Healthline.

“But I would be very cautious in then interpreting this as vitamin D is contributing to an increase in VO2 max in a causal or direct manner. It may simply be that people who are doing more physical activity are more fit, and people who are doing more outdoor physical activity have higher vitamin D levels, but not that vitamin D levels are directly related to fitness.”

People with moderate to high vitamin D levels may have the greatest cardiorespiratory fitness.

However, you shouldn’t consume large doses of vitamin D supplements to increase your supply of this important nutrient.

Instead, try to eat vitamin D-rich foods and get a sensible amount of sunshine exposure. (Talk to your dermatologist if you have a history of skin cancer.) Take a supplement only if directed to by your doctor.

If you aren’t sure what your vitamin D levels are or should be, request a test from your doctor. Together, you can review the results and consider options for increasing your levels if they’re low.